The present invention pertains generally to devices for controlling the speed of a single phase motor. More particularly, the present invention pertains to controllers for controlling motor speed in air conditioners and heat pumps. The present invention is particularly, but not exclusively, useful as a two-speed motor controller for reducing the energy consumption of air conditioning and heat pump applications.
A large percentage of air conditioners and heat pumps in residential applications are driven by single phase, permanent split capacitor (PSC) motors. Although PSC motors can be designed for high efficiency, the high efficiency is generally only realized for one load condition. In fact, the efficiency drops off rapidly as the load on the motor decreases from the optimal load. Unfortunately, the motor in an air conditioner works most of the time at fractional load, and accordingly, a considerable amount of energy is wasted.
In air conditioners and heat pumps, it is known that varying the capacity of the compressor to match the load can be used to save a considerable amount of energy. The capacity, in turn, can be varied by varying the speed of the motor driving the compressor of the air conditioner or heat pump. It is also known that by using two motor speeds (e.g. full speed and half speed), most of the energy loss due to fractional loading can be eliminated. However, unlike fans and blowers where the torque requirement goes down significantly as the speed is reduced, for compressors the torque requirement remains fairly constant or goes down only slightly with decreases in speed. Thus, to be effective, a two-speed motor for a compressor must provide a relatively high level of torque at both low and high speeds.
Several approaches have been suggested to control a single phase motor at two or more operational speeds. In a first approach, multiple taps in the main or auxiliary windings of the motor are used. In another approach, the input voltage to the motor is reduced by wave chopping using solid-state switches. Unfortunately, both of these approaches result in relatively low efficiency and significant harmonics on the utility side.
In still another approach, single phase inverters using Insulated Gate Transistors and microprocessors are used to produce a single phase reduced frequency signal. The reduced frequency signal is then passed through the main and auxiliary windings (including the capacitor) to operate a PSC motor at low speed. While this technique may provide a relatively high efficiency when torque requirements at lower speeds are minimal, the use of single phase inverters is unsuitable for compressors where the torque requirement remains fairly constant as speed decreases. Specifically, for controllers using single phase inverters, torque drops off significantly with decreases in motor speed due to the auxiliary winding capacitor that is tuned for operational efficiency at the higher speed. Additionally, controllers using single phase inverters are relatively expensive and highly complex, and generate relatively high harmonics and peak currents which can cause problems for the utility and adversely affect nearby devices (e.g. televisions, etc).
In light of the above, it is an object of the present invention to provide a speed controller for a single phase motor that can be used in air conditioning and heat pump applications. It is another object of the present invention to provide a speed controller for a permanent split capacitance motor that produces a relatively high torque at relatively low motor speeds. It is yet another object of the present invention to provide a speed controller for a single phase motor which draws a nearly sinusoidal input current (i.e. from the utility) without generating harmonics on the input line, and does not generate high peak currents. Yet another object of the present invention is to provide a speed controller for a single phase motor that can transition from a first speed to a second speed in response to a signal from a thermostat.
The present invention is directed to a speed controller for a single phase motor such as a permanent split capacitor (PSC) motor. Specific applications of the present invention include, but are not limited to, air conditioning systems and heat pumps. Functionally, the speed controller can be used to operate a single phase motor at a lower, more energy efficient speed during periods when the air conditioning system or heat pump is operating at fractional load.
For use with the present invention, the PSC motor includes a main winding and an auxiliary winding. Typically, the auxiliary winding of the PSC motor is connected to a capacitor that is tuned for operational efficiency at a first motor speed, n1. As described further below, the speed controller of the present invention allows the motor to be operated at the first speed, n1, and at least one lower speed, n2.
In functional overview, the controller causes the motor to operate at the high speed, n1, by passing an AC current having a fixed frequency,f1, through both the main and auxiliary windings of the motor. On the other hand, to operate the motor at the low speed, n2, the controller passes AC waveforms having reduced frequency, such as f1/2, through the main and auxiliary windings. In low speed mode, the controller passes a waveform through the auxiliary winding that bypasses the auxiliary winding capacitor and is shifted in phase by 90 degrees from the waveform passed through the main winding. In one embodiment, an AC waveform having a voltage, V1, of approximately 230V and a frequency,f1, of approximately 60 hertz is used to operate the motor at the high speed, n1. In this embodiment, AC waveforms having a voltage, V2, of approximately 115V and a frequency, f2, of approximately 30 hertz are used to operate the motor at the low speed, n2. The reduced voltage at the low speed prevents current saturation in the motor.
To produce the waveforms described above, the controller is connected to an AC power source. For use with the present invention, the AC power source can be a split source having output terminals A, O and B wherein the signal across terminals AO differs in phase from the signal across terminals OB by approximately 180 degrees. For use with the embodiment described above, a suitable split source has a frequency of approximately 60 hertz and a voltage, VAB, of approximately 230V with split voltages, VOA and VOB, of approximately 115V each. To operate the motor at the high speed, n1, the controller places both the main and auxiliary windings across the terminals AB.
To operate the motor at the low speed, n2, the controller generates reduced frequency waveforms from the split source. More specifically, the controller uses selected half cycles from the signals generated across terminals OA and OB to produce the reduced frequency waveforms. For example, to energize the main winding for low speed motor operation, the controller first passes a half cycle originating across terminals OA through the main winding, then skips a half cycle, then passes a half cycle originating across terminals OB through the main winding, then skips a half cycle. This sequence of selected half cycles is continued to pass a reduced frequency waveform having a frequency, f/2, through the main winding (note: f is the frequency of the signal originating across terminals OA and OB). Thus, for the split source described above, the reduced frequency waveform has a frequency of approximately 30 hertz and a voltage of approximately 115V.
As indicated above, in low speed mode, the controller passes a waveform through the auxiliary winding that bypasses the auxiliary winding capacitor and is shifted in phase by 90 degrees from the waveform passed through the main winding. To energize the auxiliary winding in this manner, the controller first skips a half cycle, then passes a half cycle originating across terminals OB through the auxiliary winding, then skips a half cycle, and then passes a half cycle originating across terminals OA through the auxiliary winding. This sequence is continued to pass a reduced frequency waveform having a frequency, f/2, through the auxiliary winding. The auxiliary winding sequence is synchronized with the sequence described above for the main winding to ensure that the waveform passed through the auxiliary winding is shifted in phase by 90 degrees from the waveform passed through the main winding.